Summary: Embryonic stem (ES) cells have the potential to differentiate into progenitor cells that are committed to a given lineage. There has been an explosion of interest in using ES cells, or their committed progenitor offspring, to repair or replace damaged tissues, yet the immune response to such cells across histocompatibility barriers has been poorly defined. This project examined the immmune response to ES cells and their committed progenitor offpring. The goal is to genetically engineer ES cells to inhibit graft rejection while providing tissue for repair. ES cells, embryoid body cells (EB) and neural progenitor cells (NP) fail to express MHC molecules and so are killed by NK cells in both syngeneic and allogeneic mice with lysis highly dependent on LFA-1/ICAM interactions. However, the response in syngeneic mice is weak and injection of ES cells into syngeneic mice yields tumor cells which progress and kill the mice . In contrast, embryonal carcinoma cells (EC), which arise in syngeneic animals injected with ES cell populations are not killed by NK or LAK cells. The failure to kill EC cells appears to relate in some EC cells to lack of expression of ICAM and in others, an increased expression of both classical and non-classical class I MHC molecules. The mechanism by which EC cells develop from ES cells is also being explored. Because of the tumorigenic potential of ES cells, prevention of rejection cannot be aimed at ES cells, but will instead be aimed at non-tumorigenic committed progentior or mature differentiated cells. To protect such committed or differentiated cells from rejection, our strategy is to clone protective molecules, such as Fas-ligand, IL-10 or TGF-beta into ES cells and place them under control of a lineage specific promoter, so that they will not be expressed until the cell assumes a differentiated (non-malignant) phenotype. We are exploring use of lentiviral vectors equipped with neural specific promoters to do so and are investigating the potential of such NP cells to facilitate healing in a murine model of multiple sclerosis (EAE). Additionally, we may clone into an ES cell line a suicide gene expressing an enzyme under control of an ES cell specific promoter. Tsuji, K. Mason, K. Mcfarland, H., Lawler, A., and A Rosenberg. Immune responses to murine embryonal stem cells, embryonal carcinoma cells and neural progenitor cells. In preparation.